Aqueous slurries of reactive calcareous and siliceous materials in appropriate proportions will react under certain conditions of temperature and pressure to form various crystalline hydrous calcium silicates. When small proportions of suitable fibrous materials, such as certain types of asbestos or cellulosic fibers, are present these crystals can be made to deposit from the reactive slurry as a rigid mass of interlocking crystals from which excess water can be removed to leave a dried polycrystalline body having a substantial proportion of its volume comprised of intersticial voids. When properly controlled, such processes can produce polycrystalline bodies in which the thermal conductivity is sufficiently low by virtue of the small size and uniform distribution at the intersticial voids and in which the mechanical strength and rigidity are sufficiently high by virtue of the interlocking of the crystals that the product is useful as thermal insulating material. Various fillers, reinforcements and/or opacifiers can also be incorporated.
The properties of such thermal insulating materials, and in particular the upper temperature limits at which they can be used in practice, will depend in considerable part on the chemical composition and crystalline form of the hydrous calcium silicate, e.g., tobermorite or xonotlite. For the highest use temperatures it is desirable that the hydrous calcium silicate in the final product be principally in the form of xonotlite crystals. The selection of the reactive calcareous and siliceous materials, their relative proportions and the reaction conditions for obtaining predominantly one or another of the various crystalline hydrous calcium silicates are well known in the art.
The apparent density of the final product has in the past been controlled largely by regulating the proportion of water in the aqueous slurry when the induration of the material commences since, except for the water of hydration incorporated in the hydrous calcium silicate, the water is ordinarily removed from the product after its induration to final rigid shape and size. In some processes (so-called filter press processes), a portion of the aqueous slurry medium is expressed from the reactive mixture at an intermediate stage where sufficient reaction has taken place to produce a self-supporting but compressible gel. In such processes the proportion of water determinative of the final apparent density of the product has been that remaining after the portion to be pressed out from this intermediate gel was expressed therefrom. In another class of processes (so-called pan or casting processes), the slurry is merely poured into pan molds of the desired shape and indurated under the appropriate conditions, the final apparent density being largely determined by the proportion of water orginally present in the reactive slurry.